Design of Investment Casting Process for ZL208 Aluminum Alloy End Cap

Abstract：The end cover of a certain aviation air pressure regulator is made of ZL208 aluminum alloy and produced through investment casting and silica sol shell making process. By designing a reasonable pouring system, using a silica sol shell making process, and adopting steam dewaxing and appropriate roasting, smelting and pouring processes, as well as heat treatment processes, castings with high dimensional accuracy and good internal quality have been obtained, with a product qualification rate of over 85%. This process method is not only easy to operate, but also environmentally friendly during shell making, making it widely used.

The product is an aviation air pressure regulator end cover, which operates in a complex environment and requires high heat resistance to withstand the impact of high temperature and high pressure loads. By analyzing and selecting ZL208 aluminum alloy for production. However, the structure of the product is relatively complex, with significant changes in wall thickness, and the casting performance of ZL208 aluminum alloy is poor [1]. When using the original gypsum mold, ethyl silicate water glass shell investment casting, defects such as looseness, choking, and insufficient pouring [2-3] appeared, and the product qualification rate was only 30%, which also caused serious environmental pollution [4]. This study analyzes the structure and materials of the product, and decides to study the silica sol shell investment casting process of the product. By designing a reasonable casting process, the internal and external quality of the product can be improved, the qualification rate of the product can be increased, and environmental pollution during shell making can be reduced.

1. Casting structure and alloy characteristics

1.1 Casting Structure

The end cover product of the air pressure regulator is shown in Figure 1. The contour dimensions of this product are 134 mm x 98 mm x 68 mm, with a complex structure and significant changes in wall thickness. The thickest part has a wall thickness of 22 mm and the thinnest part is 3 mm. During the casting process, defects such as cold shuts and insufficient pouring are prone to occur at the 3 mm wall thickness.

1.2 Alloy characteristics

The ZL208 alloy used for the end cover of the pressure regulator is ZAlCu5Ni2CoZr, which meets the HB 962-2001 standard [5]. Its chemical composition is shown in Table 1. This alloy is a high-strength and heat-resistant aluminum alloy, containing alloy elements such as Cu, Mn, Ni, Co, Zr, Sb, Ti, etc. Its composition is relatively complex. After T7 treatment, it can form complex compound phases that exist at grain boundaries, prevent grain slip, and improve thermal stability. Its working temperature can reach 400 ℃, but its mechanical properties at room temperature are low and its casting performance is poor.

2. Pouring system design

The gating system not only plays a role in guiding and filling the alloy liquid during investment casting, but also affects the temperature field during casting solidification. Defects such as porosity and porosity are closely related to the design of the riser. The original pouring system used a top pouring method, which caused local overheating and unstable filling, and was prone to oxidation and slag inclusion, resulting in defects such as choking and looseness. To solve the above problems, during the trial production process, a riser was placed on the upper part of the thickest part of the casting, and a combination of bottom and side gating was used. The horizontal gating was used to connect the gating and vertical gating, as well as 5 internal gating channels. The product gating system is shown in Figure 2. Increasing the number of internal runners not only reduces the pouring temperature appropriately, disperses heat, and reduces the occurrence of loose defects caused by local overheating, but also makes the entire filling process fast and stable, reducing the occurrence of insufficient pouring defects. And cold iron was added to other thick areas, significantly reducing porosity defects.

3 Precision casting process

3.1 Making wax molds

3.1.1 Pressing wax molds

C-162H medium temperature wax is used as the mold material, and a vertical wax injection machine is selected as the equipment. The main process parameters of the wax injection machine are set as shown in Table 2. Prepare the mold, assemble the molding in order, adjust the wax injection port to align with the wax injection nozzle of the equipment, and inject wax. After the filling is completed, cool the mold for about 1 minute before removing the wax mold and immediately cool it in room temperature water.

3.1.2 Repair wax pattern

Clean the gaps and wax injection ports to ensure a smooth and complete surface of the wax mold, and measure the wax shape size with a caliper to ensure that the wax mold is qualified.

3.1.3 Wax mold assembly welding

When welding, first weld the inner gate and wax mold, then weld the transverse gate, vertical gate, and riser. The connection between the gating system and the wax mold will have an R2-R5 arc transition. Clean the module with 1% soap powder solution to remove impurities such as oil and wax on the surface of the module. Then, rinse with clean water to remove residual soap solution. Blow with an air duct and place it on a wax rack to dry for later use. The wax mold after assembly welding is shown in Figure 3.

3.2 Preparation of silica sol shell

3.2.1 Preparation of Coatings

Corundum powder coating is composed of binder, corundum powder, wetting agent, and defoamer [6], with a viscosity requirement of 25-40 seconds. The adhesive is silica sol with a concentration of 30%; Corundum powder particle size ≥ 320 mesh; Coating ratio: silica sol (kg): corundum powder (kg): wetting agent (mL): defoamer (mL)

10:20-30:20:10-20. When preparing the coating, silica sol should be added first, the mixer should be turned on to rotate, and then wetting agent and corundum should be added sequentially

Mix the powder and defoamer evenly and measure the viscosity using a Jens cup. If the viscosity is too high, add distilled water. If it is too low, add powder for adjustment.

Mullite powder coating is composed of binder and Mullite powder [7], with a viscosity requirement of 10-22 seconds. The adhesive is silica sol with a concentration of 30%; Molite powder particles

Degree ≥ 320 mesh; The coating ratio of silica sol (kg): mullite powder (kg) is 10:20-30. In order to improve the breathability of the shell and reduce the surface area of the casting

Surface oxidation can be achieved by adding an appropriate amount of graphite powder to the mullite powder coating slurry.

3.2.2 Shell making

The first layer is coated with mortar and sprinkled with sand. Remove the module from the mold frame, slowly immerse it in the coating slurry at an angle of about 30 °, rotate slightly, then quickly remove it, rotate again to make the slurry flow on the module and drip off excess slurry, evenly apply a thin layer of slurry on the module, and then flip the coated module in multiple directions in a 90 mesh sand shower machine to evenly apply a layer of corundum sand on the surface. After sprinkling sand, hang it on the mold frame for natural drying. The second layer is coated with mortar and sprinkled with sand. In order to increase the strength of the mold shell and prevent sand loss, the mold shell is first immersed in the silica sol solution before hanging the slurry. Then, the immersed mold group is coated with a second layer of paint, and the hung slurry mold group is placed in a floating sand machine containing 30-60 mesh mullite for sand application. After sand application, it is air dried.

Use the same method to coat the third, fourth, fifth, and sixth layers of paint with slurry. The technical requirements are shown in Table 3. In order to enhance the strength of the mold shell and prevent the outer layer from falling sand, only slurry is applied to the sixth layer without sanding.

3.2.3 Dewaxing

Clean the gate end face, then place the end face in boiling water and shake it. Use the XHDJ series electric steam dewaxing kettle for dewaxing, with a dewaxing steam pressure between 0.6 and 0.8 MPa. Depending on the structure of the product, the dewaxing time of the shell is controlled at (10 ± 0.5) minutes. Check the quality of the shell after dewaxing, shell

The wax in the mold should be removed completely, and the sprue cup should be neat and free of cracks. For shells with minor cracks, paint can be used for repair to prevent fragmentation, peeling, or

Shells with cracks exceeding 0.5 mm should be scrapped.

3.2.4 Baking

Shell roasting is used to remove residual wax, moisture, and their volatiles. This study used a trolley type resistance furnace to roast the shell, which was loaded at temperatures below 500 ℃ and heated linearly to (820 ± 10) ℃ for insulation(2 ± 0.5) hours.

3.3 Melting and pouring process

3.3.1 Raw and auxiliary materials

The material of the end cover is ZL208. The raw materials are cast aluminum alloy ingots, remelted ingots, and intermediate alloys (AlCu50 AlMn10、AlSb4、AlCo5、

ALNi10, AlZr4, AlTiB4, AlTi5) and pure aluminum ingots, with a remelted ingot dosage not exceeding 80% (impurity Fe ≤ 0.4% in remelted ingots); Refining agent is

Hexachloroethane (C2Cl6) and argon gas are combined for refining. Hexachloroethane accounts for 0.6% of the weight of the alloy liquid, and the argon gas flow rate is 10 L/min.

3.3.2 Alloy melting

Clean the rust, residual oxides, sand particles and other dirt on the crucible and tools, preheat the crucible and tools to 500 ℃, turn dark red, and then evenly apply a layer of paint (25% talcum powder+6% water glass, with a remaining amount of water). The coated tools should be preheated before use, with a preheating temperature of 500 ℃ and a time of ≥ 1 hour.

Add pre made alloy ingots, remelted ingots, and intermediate alloys (AlMn10, AlCo5, ALNi10, AlZr4, AlTi5) prepared according to requirements into the furnace. After all are melted, stir slightly and raise the temperature to (740 ± 5) ℃. Add AlCu50 and AlSb4 intermediate alloys and stir for 3 minutes after melting; Then heat the alloy liquid to (770 ± 10) ℃ and stir for (6 ± 1) minutes; Cool to 740-750 ℃, add preheated aluminum titanium boron for refinement treatment, and stir for 3 minutes.

In order to ensure the quality of degassing, a combined refining process using hexachloroethane and argon degassing machine is adopted. During the refining process, the alloy liquid temperature is 730-740 ℃, and hexachloroethane is divided into 3-5 parts and pressed into the alloy liquid using a bell shaped cover. Then refine with argon gas at a flow rate of 10 L/min for 15 minutes.

After refining with argon gas, let it stand for 15 minutes to skim the slag, and then conduct a pre furnace degassing inspection. Pour the alloy liquid into the iron mold with a small spoon and observe its surface condition. If there are no bubbles emerging during the solidification of the alloy liquid, it indicates that the gas inside the alloy is qualified. During the solidification of the alloy liquid, many small bubbles emerge, and after solidification, many small pits or protrusions appear, indicating that the alloy has not been degassed and meets the standard, and degassing needs to be carried out again. Adjust the alloy liquid to 710-720 ℃ and prepare for pouring.

3.3.3 Alloy casting

During the experiment, different pouring temperatures and shell preheating were used temperature combination. Through experimental verification, it has been found that lower pouring temperatures and shell preheating temperatures make it difficult for castings to fill the mold, and insufficient riser shrinkage can result in inadequate pouring foot and cold insulation defects; Higher pouring temperature and shell preheating temperature can lead to long solidification time of castings, resulting in porosity defects. When the number of shells is more than 4, there are many defects such as cold shut and insufficient pouring in the casting, as shown in Figure 4. After testing, reasonable process parameters were determined, as shown in Table 4.

The product pouring adopts gravity pouring. When taking out the mold shell from the furnace, keep the gate facing downwards, pour out any debris that may be present in the mold cavity, and use a dry air duct to blow out the mold cavity. In order to fix the position of the mold shell during pouring, sand should be used to lay the ground to assist in flattening the mold shell. When pouring, align with the gate, smoothly, evenly, and continuously inject the molten metal into the mold, and keep the gate full without interruption. Stop pouring when pouring to 1/2 of the riser, and depending on the degree of solidification of the riser, re scoop the molten metal into the riser.

The casting should be taken out of the box for ≥ 8 hours. After the casting is taken out of the box, the residual shell should be cleaned, the riser should be cut off, the inner cavity should be cleaned with a high-pressure water gun, the burrs and aluminum beans should be cleaned, and the surface should be polished before initial inspection. There should be no defects such as cold shuts, cracks, looseness, and shrinkage on the surface. According to the above process parameters, 27 products were produced in each furnace, and 24 were qualified. The product qualification rate reached 88.9%. The casting has undergone low magnification inspection and the pinhole degree has reached level I.

4 Heat treatment process

After passing the inspection, the product is subjected to T7 heat treatment, and the equipment is equipped with a vertical rapid quenching furnace. The quenching temperature is (540 ± 5) ℃, the insulation time is 5.5-6 hours, and the interval between the parts being taken out of the furnace and quenched into water is not more than 15 seconds. The residence time of the castings in water shall not be less than 2 minutes. After quenching, the castings are placed in an aging furnace for artificial aging, with an aging temperature of (215 ± 5) ℃ and a holding time of 16-17 hours (Figure 5). Managerial testing showed a tensile strength of 284 MPa and an elongation of 2.6%.

5 Conclusion

(1) During the shell making process, the drying time of the first layer must be controlled to dry for more than 20 hours without wind, and during the shell making process of the second to sixth layers, it must be dried for more than 6 hours under strong wind conditions;

(2) When the pouring temperature is controlled at (715 ± 5) ℃, the shell temperature is controlled at (450 ± 5) ℃, and the number of shells is ≤ 4, it can effectively avoid defects such as cold shut and looseness in the product;

(3) When the quenching temperature is controlled at (540 ± 5) ℃, the holding time is 5.5-6 hours, the transfer time is ≤ 15 seconds, the aging temperature is controlled at (215 ± 5) ℃, and the holding time is 16-17 hours, the tensile strength of ZL208 alloy is ≥ 280 MPa and the elongation is ≥ 2%;

(4) By designing a reasonable pouring system and strictly controlling the production process of silica sol shell making, the dimensional accuracy and internal quality of the product have been effectively improved, resulting in a qualification rate of over 85%. However, the qualification rate still needs to be improved, and the subsequent process methods need to be improved, such as using vacuum casting and other processes, to enhance product quality and qualification rate.